Refining of lead alloys



Patented June 13, 1933 UNITED STATES PATENT OFFICE THOMAS E. HARPER, J R., AN D GUSTAVE REINBERG, JR., 0]? LA OROYA I ERU, ASSIGN- ORS TO CERRO DE PASCO COPPER CORPORATION, OF NEW YORK, N. Y., A CORPO- RATION OF NEW YORK REFINING OF LEAD ALLOYS No Drawing.

This invention relates to an improvement in the process of refining lead bullion by electrolysis, whereby this process may be applied to bullions containing a relatively high percentage of impurities, and especially to bullions or alloys containing a high percentage of bismuth.

The electrolytic process of refining lead bullion in an electrolyte of fluosilicic acid lead fluosilicate (Betts process) has long been successfully applied to bullions containing not more than 2 to 2 70 of impurities. In such procedure, it has been the practice to-use anode plates of a thickness about 1 to 1 inches which were replaced at intervals of from 8 to 10 days. WVlien attempts were made, however, to refine lead bullion containing a higher percentage of impurities by this procedure, it was found that the acid loss from the electrolyte was unduly increased, and the lead deposited at the cathode was contaminated by impurities, even when the currentdensity was decreased to an impractically low figure.

It is an objectof the present invention to provide an improvement in the process for electrolytically refining lead bullion, by which a bullion containing higher percentages of impurities may be refined on a commercial scale. It is also an object to provide an improved method by which high grade lead may be electrolytically separated from bismuth in lea d alloys containing relatively high percentages of bismuth. It is a further object of the invention to provide animproved procedure for washing the slimes formed uponthe' anode during the electrolytic separation. Other objects will become apparent.

Infollowing the procedure of the present invention, impure bullions containing more than 3% of impurities may be refined electrolytically with a normal loss of acid, employing the usual current densities, and with the production of refined lead of satisfactory purity for commercial sale'as the highest grade refined lead. In other words, the improved process makes bullions containing less than 97% lead and from 3 to Application filed September 24, 1931.

every four days.

porous, adherent blanket or slime.

Serial No. 564,965.

commercial basis and represents an extension of the electroyltic refining process into fields in which it was previously considered commercially inoperable.

In carrying out this improved procedure, the impure lead bullion is cast into anodes and these anodes, together with the cathode I sheets of pure lead, are placed in an electrolyte of lead fiuosilicate and free fluosilicic acid. Current is passed through the cell or cells while the electrolyte is circulated and agitated to give ample and relatively vigorous circulation between the several electrodes. In following our improved procedure, it is desirable to agitate the electrolyte more vigorously than practicable in the prior practice of treating lead bullion containing low percentages of impurities, for example, with an anode containing about 80% lead the agitation or circulation may be about twice as rapid as that ordinarily used in practicing the prior art process with an anode containing about 98% lead. This circulation maybe accomplished by any suitablemeans, for example, by a pump operated to recirculate the electrolyte from the electrolytic cell or by a mechanical stirring device, or by bubbling air into the bottom of the cell. The cathode sheets are changed at convenient intervals, for'example, abont During electrolysis the lead goes to the cathode and the impurities remain attached to'the face of the anode in the form of a i The voltage drop across this blanket or slime 'will increase as 'the'electrolysis proceeds and an important feature of the present process comprisesthe removal of the anodes after aboutone to three days, or when the voltage drop across this'slime blanket has increased to-not over about .2 of a volt. This increase in potential across the slime blanket is roughly equivalent to an increase of about .3 of a volt in the over all voltage across the cell, although the latter "alue may vary somewhat, depending upon the characteristics of the cell, without affecting our improved procedure.

50%} bismuth amenable to treatment on a Alternatively, the anodes may be cast thicker and after removing a layer of impurities at the end of, for example, one to three days, may be replaced in the cell; with this method of operation, however, the anode surface must be cleaned very carefully since there is danger that a small amount of impurities may be left on that surface and be oxidized and pass into solution, from which it will be deposited on the cathode. It is preferred, therefore, to cast the anode just thick enough to leave a solid unbroken sheet of metal at the center after the corrosion has proceeded to such an extent that the slime blanket voltage is about .2 of a volt. After washing as hereinafter described, the impurities may be removed from the sheet by scraping or other suitable means, after which the sheet may be remelted, and added to the metal to be cast into new anodes.

Another important feature of our invention resides in the counter-current dipping of the anodes, whereby the corroded anodes are successively brought into contact with washing water of decreasing concentration. The electrolyte mechanically held within the interstices of the blanket of impurities is removed therefrom by this washing process through the osmosis and diffusion of the electrolyte into the more dilute solution. The electrolyte removed in this manner may be re-used by adding the most concentrated washing solution to the electrolyte in the cell. If desired, this solution may be evaporated to remove excess water before adding it to the electrolyte in the cell. If the operation is continued over a long period of time, it may be necessary to add some free acid to the evaporated solution to bring the total acid concentration up to the desired value.

An important advantage of this countercurrent washing of the anode blanket or slimes resides in the fact that the slime blanket which carries a considerable percentage of soluble lead fluosilicate, is immersed in asolution of sufficient concentration of fluosilieic acid to prevent hydrolysis and precipitation of basic lead fluosilicate or lead fluoride within the pores of the blanket. By following our procedure, the lead fluosilieate diffuses into the acid solution and is returned to the cell with the electrolyte.

As a specific example of operations according to this improved procedure, anodes of about inch in thickness were made up from lead bullion which assayed as follows:

Lead 80.6 Bismuth 12.0 Copper .25% Antimony 2. Arsenic c 3. 30% Silver 4:7. 1 ozbper ton These anodes were placed in electrolytic cells containing a fiuosilicic acid-lead fluosilicate electrolyte of the following concentration:

Grams per liter Lead 9 Free acid 84 Total acid 139 The electrolyte was continuously circu lated so as to obtain an uniformly active flow of electrolyte over the electrodes. The annode current density was maintained at about 14 to 15 amperes per square foot and the anodes were replaced about every 24 hours, during which time the potential drop across the cell gradually increased by about .3 or .4 of a volt. About one pound of glue was added to the electrolyte per ton of lead deposited.

The average results of the operation as above described for 18 days were as follows: bismuth in cathode, trace to 003%, acid lost per ton of lead deposited 6 lbs, current efliciency 90%.

Nlv'mes assay Percent Lead 1.8 Bismuth 55.6 Copper 1.1' Antimony 12.6 Arsenic 15.9 Silver 0.80

Thelead produced by the above procedure was very pure and came well within the class of the highest grade of refined lead, no other impurities being present in the lead in greater concentration than the bismuth reported above.

The slime blankets were porous and sufficiently tenacious to resist rupture during the removal of the anodes from the cells and the counter-current washing operation to which the used anodes were subjected. In this washing the anode was dipped successively into each of a series of tanks so that the first washing tank would contain the highest concentration of acid and lead fluosilicate, the concentration of the remaining tanks gradually decreasing. The concentrated electrolyte from the washing operation was reintroduced into the cell without evaporation and without adding any new acid to it and the concentration of the electrolyte after operations according to the above example for 48 days, was as follows:

or only slightly higher than the normal acid loss and to maintain constant the lead conperiod of 12 days according to the above procedure and with substantially the same composition of anodes, but using asphalt lined apparatus instead of lead lined apparatus, as in that example, the initial and final electrolytes analysed as follows, the figures being corrected to equal volumes.

Initial Final Leach... '77.? 77.4 Free acid 91.5 80.0 Total acid 145. 6 134. 0

As another example of our improved procedure, anodes assaying as follows:

Percent Lead 83.1 Bismuth 12.7 Arsenic 2.1 Antimony 1.7

were placed in a fiousilicic acid-lead fluosilicate bath containing Grams per liter Lead 84.8 Total free and combined flousilicic acid 121.1

Free fiousilicic acid 24 hrs. 48 hrs. 72 hrs. 96 hrs.

Voltage drop, across cell (volts) .35 44 .54 .70 Voltage drop, anode to surface of slime (slime blanket) (volts) .04 .12 .18 .50 Weight of cathode (grams). 624 1400 1900 2670 Bi in cathode lead 001 .002 .007 .47 Pb in washed slime 1.0 1.5 2.1 1. 2

rl'ilfllJ/Slfi of solution in interstices of slime blanket i Pb grams/liter 342 440 496 487 Total HzSiFu grams/liter 252 325 357 392 Free l'IzSiFe grams/liter 14 20 11 52 The countercurrent dipping described above is an important improvement in the ,washing water at a rate equal to that of removal of concentrated Washing the water from the last tank. Instead of dipping the anodes into-separate tanks during the washing operation, this may be accomplished by continuously moving a series of anodes through a tank against a constantly moving current of washing water, the dilute washing water being introduced at the end opposite that at which the anodes are introduced and the concentrated washing water being removed at the other end, provision being made by bafiiing or otherwise to avoid stratification of the liquid in the tank.

Other electrolytes maybe used in place of the fiuosilicic acid-lead fluosilicate, for example, those comprising fluoboric acidlead fluoborate or perchloric acid-lead perchlorate, or lead salts of organic sulphonic acids, although the electrolyte given-is generally preferred. The anodes may be allowed to remain in operation for a longer time than described above and the time will vary with the concentration of impurities in the anode, but should not be for longer than would result in the solution of the impurities in the anode. The preferred practice, with usual cell constants, is to remove the anodes when the voltage drop across the slime blanket reaches about .15 to .2 volts.

It is apparent that the above procedure may be modified to meet the particular needs of the operation to which it is applied and it is not intended to restrict it to the particular example given. The terms used in describing the invention have been used in their descriptive sense and not as terms of limitation and it is intended to include'within the scope of the appended claims all equivalents of the procedures described.

This application is a continuation in part of our copending application Serial No. 505,976, filed December 31, 1930.

lVhat we claim is:

1. A method for electrolytically refining lead bullion containing more than 3% of bismuth, comprising passing an electric current at a current density of above about 14 amperes per square foot through the lead bullion as an anode in an electrolyte, and re moving the anode and the attached slime blanket when the voltage of the cell has increased by about .3 of a volt.

2. In the electrolytic refining of lead alloy containing more than about 3% of bismuth, the steps comprising passing anelectric current through the lead'alloy as an anode in an electrolyte at an anode current density of about 13-25 amperes per square the anodes when the cell voltage increases by not over .3 volts, subjecting the anodes with their attached slime blankets to countercurrent washing and returning the mos-t concentrated washing water to the cell.

3. In the electrolytic refining of lead alloy, the steps comprising passing an electric current through the lead alloy as an anode in an electrolyte, removing the anode when the voltage drop across the slime blanket is increased to not over about .2 volts, subjecting the anode with its attached slime blanket to a countercurrent washing and returning the concentrated washing water to the electrolyte. v 4. In the electrolytic refining of lead al- I loy, the steps comprising passing an electric f 'current through the lead alloy as an anode "in an electrolyte containing fluosilicic acid and lead fluosilicate, while circulating the electrolyte, removing the anode after the Voltage of the cell has increased by about .3 of a volt, and dipping the anodes in washing solutions containing gradually decreasing percentages of fiuosilicic acid.

5. In the electrolytic refining of lead alloy, the steps comprising passing an electric current through the lead alloy as an anode in an electrolyte while ,circulating the electrol'yte, removing the anode after the voltage, of the slime blanket has increased to about .15 to .2 of a volt, and dipping the anode in a washing bath containing sulficient fluosilicic acid to prevent deposition of lead compounds Within the slime'blanket, and thereafter washing the slime blanket to complete the removal of-the acid.

6. A method for electrically refining lead. bullion containing more than 3% impurities to produce cathode lead containing not more than .007 bismuth, comprising passing an electric current at an anode current density of not less than about 14 amperes per square foot through the lead bullion as an anode in an electrolyte and removing the anode and the slime blanketwhen the voltage drop across the slime blanket reaches not over about'.2 volts.

7. In the electrolytic refining of lead alloy, the steps comprising passing an electric current through the lead alloy as an anode in an electrolyte, and subsequently subjecting the anode with its attached slime blanket to aslolution containing suflicient fluosilicic acid to prevent deposition of insoluble compoundsof lead and fluorine within the slime blanket and to; permit diffusion of the entrained liquid.

8. A method for electrically refining lead bullion, comprising passing an electric current through the lead bullion as an anode in an electrolyte until the voltage drop across the cell increases by not over about .3 volts, and thereafter subjecting the anode" with its attached slime blanket to a solution containing sufiicient fiuosilicic acid to prevent formation of insoluble compounds of lead and fluorine within the slime blanket and to permit diffusion of the entrained liquid, and thereafter subjecting the slime blanket to further washing. 

